The present invention relates to railroad cars, and particularly to a railroad car defining a well, for carrying intermodal cargo containers stacked one upon another.
In order to obtain better overall fuel efficiency in carrying containerized cargo over long distances, intermodal cargo containers are often carried on railroad cars in situations where rail transport does not interfere with achievement of required delivery dates. Multi-unit articulated railroad cars can be built strong enough to carry containers stacked one atop another, since containerized cargo is usually not so dense that car weight combined with the weight of loaded containers will exceed the maximum weights which can be imposed upon railroad tracks. When containers are stacked two containers high on a railroad car, however, overall height is a definite consideration, since only a limited height such as 20'2", or in some cases less, is available on some main railroad track lines, and it is therefore desirable to provide a car capable of carrying containers stacked two high yet with the maximum height of the loaded car as low as possible, even when carrying empty containers.
Similarly, the dimensions of a car capable of carrying containers must be within width restrictions resulting from signals and other equipment located alongside tracks. The width of a car midway between its trucks must be narrower when the truck spacing is greater, in order for the car to remain within the available clearance envelope as curved track is negotiated. Lateral clearance is particularly a problem in the design of railroad cars intended to carry containers such as 48-foot-long 102-inch wide containers, rapidly becoming an accepted size in the transportation industry, since such containers are of a greater width than shorter containers, and yet a car carrying such a container must still fit within the available clearance, since the cost of modifying railway track lines to provide greater clearance is prohibitive.
Sufficient clearance must also be maintained beneath a car while sufficient stiffness of the side sill structures and the floor structures, if any, of the car must be provided, and the maximum height of the side sills must be kept low enough to permit use of the container loading cranes and associated equipment in use at container loading yards.
Intermodal cargo containers are constructed to be carried with their weight transmitted through load carrying structures normally located at the corners of shorter containers and sometimes spaced longitudinally a short distance from the corners of longer standard containers. Standard containers include vertical loadbearing structures permitting such containers to be stacked one atop another and to be interconnected to prevent separation during transport. The locations of the load bearing structures in the containers are standardized so that containers having various lengths such as 20 feet, 24 feet, 40 feet, 45 feet, or 48 feet can be supported on support structures provided at standard spacings on highway truck chassis, on railroad cars, and in container-ship holds. Such support structures must be capable of supporting the entire load of containers and the enclosed cargo, yet must fit within the limited amount of space available, which, in the case of containers on railroad cars, is defined partly by the clearance available along track lines. The problem of designing a car with sufficient strength to support cargo containers during operation of a train, where dynamic loads caused by track unevenness, car performance dynamics, centrifugal force, and wind forces are applied, is complicated by the desirability of rail carriage of wider, heavier, and longer containers, necessitating longer railroad car truck spacings, while keeping the lateral and vertical dimensions of the car within the available clearance envelope.
As a result of these competing considerations, the structure of a railroad car for carrying stacked cargo containers must be strong, shallow, and narrow, yet not too expensive to build. Others have attempted to solve similar problems in previously available railroad cars by using corner castings or weldments incorporating container support structures, and have used stiffened floor structures interconnecting the side sills of a well car, in order to provide sufficient strength to carry the loads imposed by loaded intermodal cargo containers or trailers. Conventionally accepted engineering practice has taught previously that the construction of container support structures massive enough to support the expected loading, yet remain in the space available, would require heating the metal to bend, weld, or cast the necessary structure, adding to the cost of a car.
Previous attempts to construct a satisfactory railroad car for carrying the longer, wider, containers have not been entirely successful. They have resulted in an undesirably great height of a loaded car, particularly when carrying two containers stacked one atop the other with a light load weight, or they have been unable to withstand the forces of carrying loaded containers without early failure. A further consideration in such railroad cars is that of providing lateral stability to resist rolling over, in view of the height of the center of gravity of cars loaded with cargo containers stacked atop one another, particularly where a longer container is carried atop a shorter one. Superelevation of one of a pair of rails, provided to accommodate high-speed passenger train operation, may cause problems in maintaining the stability of a freight car carrying cargo containers stacked one atop another, particularly where forces resulting from irregularities of the track are superimposed on the normal dynamic forces resulting from operating the train. It has previously been thought that the use of multi-unit articulated car construction was highly desirable, if not absolutely necessary, in such cars, because of the ability of adjacent units of one multi-unit car to stabilize one another as a car moves along railroad tracks. It was felt that a conventional single-unit car, with both ends supported on unshared trucks, lacked sufficient stability for safe operation with containers stacked one atop another. One factor in such stability of a car during operation is the tendency of trucks in previously known cars to "hunt," that is, oscillate directionally about a vertical axis of rotation with respect to the car body. Such oscillation has occurred because of the need to allow cars to negotiate curves in tracks without requiring too much power to rotate the trucks with respect to the car body, particularly when the side bearings are loaded, even though some resistance to turning is known to be desirable to avoid such oscillation, particularly for empty cars travelling at higher speeds.
Yet a further consideration resulting from the use of increasingly longer cargo containers is the need for a container-carrying car to be able to negotiate curves and hills satisfactorily, within the clearances available along the tracks, and taking into consideration the twisting forces which may be encountered by a longer car as a result of the transition of the tracks into a condition of superelevation of the outer rail in a curved length of track. The difference in superelevation between opposite ends of a long car leaves the inside front and outside rear wheels of a car entering a curve more lightly loaded than the others, making it more likely that such wheels may climb the rails.
As a result, what is needed is an improved railroad freight car for carrying large intermodal cargo containers with a resultant overall height of the loaded car which is as small as possible, yet with sufficient clearance beneath the car and with sufficient lateral clearance for operation of the loaded car on most railroad lines, and with sufficient stability for reliably safe operation of single-unit cars with unshared trucks carrying containers stacked two tiers high.